“Millet” is not a botanic term for a certain plant but rather an umbrella term for various small seeded grasses used for human consumption. In the US and Europe, proso millet (Panicum miliaceum) is the major millet consumed.

All millets belong to the order of Poales, and there to the family of Poaceae (also Gramineae or true grasses). They belong to either of the two subfamilies of Panicoideae or Chloridoideae.

Eragrostideae tribe (Chloridoideae subfamily) :

• Eleusine coracana: finger millet, mawere (ragi, nachani or mandwa in India)
• Eragrostis tef: teff

Paniceae tribe (Panicoideae subfamily) :

• Panicum miliaceum: proso millet, common millet, broom corn millet, hog millet, yellow hog, white millet
• Pennisetum glaucum: pearl millet (kambu or bajra in India)
• Setaria italica: foxtail millet, German millet (thinai, kang or rala in India)
• Digitaria spp.: white fonio, black fonio, raishan, Polish millet
• Echinochloa spp.: Japanese barnyard millet, Indian barnyard millet, sawa millet, burgu millet (kuthirai vaali, bhagar or varai in India)
• Panicum sumatrense: little millet (samai in India)
• Paspalum scrobiculatum: kodo millet (varagu in India)
• Urochloa spp. (also known as Brachiaria): browntop millet (U. ramosa, dixie signalgrass), Guinea millet

Andropogoneae tribe (Panicoideae subfamily):

• Coix: Job’s tears

 

Here are pictures of some millet varieties:

 

Sources: Wikipedia, EC137 – Proso millet in the great plains, http://plants.usda.gov/core/profile?symbol=URRA